1. Technical Field
The present invention relates to an illuminator for a film scanner and, more particularly, to a linear light source illuminator for use in a film scanner employing a linear image sensor, which is particularly suitable for scanning standard 35 mm motion picture film.
2. Background Art
State of the art illumination systems for telecine film scanners typically include light-sensitive charge-coupled device (CCD) linear arrays, which provide a serial output representing a line of a television raster, and a light source for illuminating a linear section of the film being scanned with either a scanning light beam or a line of diffuse light. For color television applications, the film scanner can include a light source that is subjected to dichroic filtration to tailor the spectral content of the light according to the film to be scanned. A light integrator transforms a light beam from a light source into a line of diffuse light emitted through an elongated aperture for illumination of the film frame. The line of light must be sufficiently uniform in intensity and diffusion along its length, and must have the red, green and blue spectral radiances for the film being scanned. As light passes through the illuminated linear section, it is color- and intensity-modulated by the image therein, and is imaged onto 3-color filtered CCD arrays as described, for example, in commonly assigned U.S. Pat. No. 5,012,346 to DeJager et al and U.S. Pat. No. 5,003,379 to Moore, Jr. et al.
The film motion provides the vertical (frame) scan, and the linear synchronized electronic cycling of the CCD array provides the horizontal (line) scan of a rasterized television signal with both chrominance and luminance components. A number of the considerations taken into account in scanning positive (print) and negative color film stocks with such telecine film scanners is set forth in the article "Optical Scanning Systems for a CCD Telecine for HDTV", by Kurtz et al in SPIE, Vol. 1448, Camera and Input Scanner Systems, 1991, pp. 191-205. As described in that article and in commonly assigned U.S. Pat. No. 4,868,383 and U.S. Pat. No. 5,155,596 to Kurtz et al, and U.S. Pat. No. 5,012,346 to DeJager et al, such film scanner linear light sources have difficulty transforming light emitted by a source lamp into a line of diffuse light of sufficient, uniform intensity for the full lamp life in operating intensity ranges required and which does not vary from moment to moment due to lamp "noise". In the '383 patent and the '346 patent, a linear light integrator is provided with a light-intensity sensor and feedback circuit for regulating the lamp power supply to dampen light output intensity fluctuations of a Xenon arc lamp. Intensity fluctuations from the targeted lamp intensity may be both temporal and spatial, as arc length and position within the lamp envelope varies and shadows caused thereby change from moment to moment. The severity of the fluctuations increases as lamp current is decreased, so that the Xenon arc lamp is preferably driven at a relatively high current and intensity. An integrating cylinder is designed to integrate the light beam from the lamp so that a line of light is emitted as uniform intensity along the length of the aperture at any given moment, regardless of spatial fluctuations and light beam intensity irregularities. Consequently, the intensity of the uniform line of light varies temporally. The temporal variation in the integrated light intensity from the target or reference intensity is referred to as illumination "noise" that is considered in measuring the illumination signal-to-noise ratio performance of the system. This noise level is reduced to acceptable levels by means of the feedback loop.
In recent designs for illumination systems for film scanners, such as for transferring motion picture film into video, much use has been made of light integrating means for providing uniform and diffuse illumination. The uniform illumination of the film assists the image transfer process by reducing the extent of the electronic pattern correction, and preserving the signal to noise ratio. Effectively, all parts of the object are illuminated identically, regardless of position. These film scanner designs are typically non-coherent systems, in which the acceptance angle of the imaging lens is appreciably smaller than the diffuse spread of illuminating light. This diffuse illumination provides so called "scratch suppression", which prevents phase artifacts, such as scratches, from being seen in the transferred image. It compensates for the light which is lost due to refraction and scattering at a scratch by providing higher angle incident light, which is then deflected off the scratch and into the acceptance angle of the imaging lens.
Much of the recent art for the design of film scanner illumination systems has used integrating cylinders to provide diffuse and uniform illumination. In such systems, the homogenizing of the light is largely a function of there being multiple scattering reflections of the light within a cavity before it exits and illuminates the film. As compared to an idealized "single pass" design, in which the integration is accomplished with relatively few surfaces, an integrating cylinder design is inefficient. Furthermore, the integrating cylinders intrinsically provide Lamberthian diffuse illumination, wherein the light is diffused out over a full 180.degree. hemisphere. However, even for scratch suppression, light diffused out beyond approximately 50.degree. is essentially wasted; as the light beyond approximately 50.degree. provides only a marginal gain in the quality of the scratch suppression for the light lost. Furthermore, such light can contribute to increased flare in the image portion of the film scanner. Thus, it would be desirable to have a "single pass" integrator for a film scanner that provided increased efficiency, both by reducing the number of surface interactions and by limiting the angular extent of the diffused light to less than 90.degree..
This increased light efficiency could then be applied in many ways that could be beneficial to the design of a film scanner, such as: allowing for the use of a lower power lamp and power supply; allowing for the use of lower cost optical elements, including the imaging lens; and/or allowing for a faster rate of image transfer.
In the past, some so called "single pass" systems have been designed using optical diffusers, such as ground glass, opal glass, ceramics, etc., to provide the means for integrating the non-uniform light source into appropriately uniform spatial and angular distributions of illumination. However, like the integrating cylinder, such diffusers often scatter light in a nearly Lamberthian manner, with little control of the angular extent of the diffusion. As a result, such classical diffusers can be highly inefficient in an optical scanner.